Superalloys are high-temperature, oxidation-resistant alloys with high strength levels. These superalloys have wide application in the aircraft propulsion industry and are also used in other industries such as automotive and chemical processing. Superalloy metallic articles are fabricated by any of a number of techniques, as may be appropriate for the nature of the metal and the article. In one common approach, metal-containing ores are refined to produce molten metal, which is thereafter cast. Ore refinement may take place separately for each of the major alloying elements, or in combination for more than one element. Elements and combinations of elements may take many intermediate forms before being melted to form the final alloy. The metal is refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or during remelting. After a superalloy of the desired composition is produced, it may be used in the as-cast form for some superalloy compositions (i.e. cast superalloys), or it may be cast and further worked to form the metal to the desired shape for other superalloy compositions (i.e. wrought superalloys). It may instead be atomized to form fine powder and subsequently consolidated and, in some cases, further worked (i.e. powder metallurgy superalloys). In any case, further processing such as joining, heat treating, machining, surface coating, and the like may be employed.
Regardless of processing route, all of these forms involve melt processing and are, as a result, subject to restrictions imposed by such processes. Melting of superalloy materials is typically accomplished using multiple melt processes in order to refine undesirable residual element content, to homogenize the overall composition, adjust the alloy content, and cast the final ingot or article. In multiple melt processes, intermediate cast electrodes are produced which serve as the input stock to the subsequent melting step. Melting processes include those which are not performed under vacuum such as electroslag remelting and electric arc furnace melting followed by argon-oxygen decarburization, and those which are performed under vacuum such as vacuum induction melting and vacuum arc remelting. Non-vacuum processes require the use of refining and protective slags during the melting process.
Additional limitations are also imposed as a result of the melting process for superalloys because of their compositions and the propensity for formation of melt-related irregularities. Alloy composition and resulting segregation issues during solidification impose practical limits on the melting rate and the maximum electrode or ingot diameters, which can be produced without gross irregularities. In order to reduce the incidence of melt-related irregularities, strict melt controls are imposed so as to monitor and control melt rate, heat input, melt temperature, electrode-to-crucible clearance, and other critical parameters. Inadequate control of these parameters can result in material with significant solidification-related irregularities, which, in turn, can reduce yield and increase production costs. In cases, such as in vacuum induction melting, where one alloy is melted in the ceramic melt crucible, and then a second alloy of a different alloy is to be subsequently melted in the same vessel, an intermediate “wash heat” is required in order to minimize alloying element contamination from one alloy to the next which may result from residual alloy material remaining on the crucible wall. This wash-heat requirement adds to the overall cost of producing high-quality superalloy material.
Irregularities may result from melting processes or as a result of subsequent forming operations. Melt-related irregularities include those related to segregation as well as those resulting from extrinsic contaminants such as air and crucible ceramics. Melting of superalloys is subject to significant solidification segregation that can result in the formation of irregularities such as freckles, eutectic nodules, and white spots. Freckles are the result of alloying element partitioning during solidification, and are most prevalent in those materials that are highly alloyed to achieve improved properties. White spots, likewise, are a result of alloying element segregation, but can also be associated with extrinsic contamination from crucible ceramics or remnant slag inclusions (dirty white spots). These melt-related irregularities can significantly degrade the fatigue resistance of the superalloy material. Melt-related irregularities can also contribute to forging-related irregularities such as cracking. Some highly alloyed materials are also more difficult to form as a result of inheriting the coarse cast structure, which can lead to additional forging-related irregularities.
Some superalloys are also produced using powder metallurgy processes to circumvent these segregation irregularity issues, particularly for large-diameter ingots, and to reduce the size of extrinsic contaminants resulting from the multiple melt process. The current powder metallurgy processes, however, require superalloy material to first be melted to produce alloy ingot, and then remelted and atomized to produce powder. These powder metallurgy processes add great expense and can still result in extrinsic contamination from crucible ceramics and slag. In addition, powder metallurgy processes are also subject to concerns related to inert gas entrapment in powder particles during the atomization process, which can lead to residual porosity in the resulting billet or component. These irregularities can degrade the fatigue properties of articles produced by the current powder metallurgy process.
Although conventionally produced superalloys possess high-temperature strength, corrosion resistance, and oxidation resistance, increasingly more severe application service conditions result in the need for further improvements in strength, temperature capability, and environmental resistance. Revolutionary improvements in these properties have not been largely possible due to compositional constraints imposed by melting and working processes. Significant improvements in corrosion and oxidation protection are required to improve the service temperature and time limitations with current alloys. These limitations may not be addressed currently or may only be addressed through application of additional coatings.
The production of some desirable compositions of superalloys may be complicated by the differences in the thermophysical properties of the metals being combined to produce the alloy. The interactions and reactions due to these thermophysical properties of the metals may cause undesired results. To cite one example, base metals such as nickel, cobalt, and iron are, in some cases, melted in a vacuum to ensure low oxygen and nitrogen contents in the final alloys. In the work leading to the present invention, the inventors have realized that the necessity to melt under a vacuum makes it difficult to utilize some desirable alloying elements due to their relative vapor pressures in a vacuum environment. The difference in the vapor pressures is one of the thermophysical properties that must be considered in alloying such base metals. In other cases, the alloying elements may be thermophysically melt incompatible with the molten base metal because of other thermophysical characteristics such as miscibilities, melting points, densities, and chemical reactivities or may have limitations in alloy content due to solidification reactions which form undesirable phase morphologies. Some of the incompatibilities may be overcome with the use of expensive master alloys, but this approach is not applicable in other cases.
As a result, the inventors have recognized in the work leading to the present invention that melting processes impose significant compositional and structural limitations on the resulting article. Incremental performance improvements resulting from processing modifications and incremental improvements in production cost reduction are still possible in a number of areas. However, in other instances the fabrication approach involving multiple melt steps imposes fundamental performance limitations that cannot be overcome at any reasonable cost. They have recognized a need for a departure from the conventional thinking in fabrication technology, which will overcome many of these fundamental limitations. The present invention fulfils this need, and further provides related advantages.